When units, such as electronic components, or the like, are manufactured using modern automated processes, a certain percentage of those units will contain defects, missing pieces or other similar imperfections. For example, when printed circuit boards (PCBs), such as those used in cellular telephones, are manufactured, a percentage of the produced PCBs will be missing components. In order to ensure quality, manufacturers typically test the manufactured units for the presence of defects. For example, after a PCB has been manufactured, optical inspections are typically made of the PCB's surface. By comparing these images to known standard images, defects can be detected. These optical inspections may be color images used to determine surface quality, gray-scale images are used to determine whether a component is present or absent on the PCB, or even three dimensional images of the PCB surface used to determine the quality of solder joints. Other types of test such as electrical conductivity or x-ray imaging may be used as well. The type of test selected is typically a function of the type of unit being manufactured, the type of manufacturing process itself, the testing environment, or other similar consideration.
Most of the testing methods used to detect defects by comparing the results of a test on a newly manufactured unit to the results of a known standard. Optical inspection tests, such as those described above, for example, compare the image of a manufactured PCB to the image of a known “good” PCB. A defect can then be detected when the image of the PCB is perceptively different from the image of the known “good” PCB. Other methods use similar methods to detect the presence of various defects. For example, during manufacture, a malfunction may cause solder joints used to affix chips onto PCBs to overflow. Such a defect will often be detected using electrical connectivity tests in which the connectivity of a newly manufactured unit is measured and compared to the values of known “good” unit. If the measured value differs by more than a preset amount, the manufactured unit is considered defective.
Traditionally manufacturing techniques use only one testing technique to detect the presence of a defect. Typically, that test was chosen because it is considered the most suitable (or accurate) given the manufacturing process, the type of unit being manufactured, and the type of defects anticipated. For example, if color imaging produces the most accurate results for detecting the presence of a component, color imaging is used. If gray-scale imaging produces the most accurate results for detecting textual codes on an integrated circuit (IC) chip, gray-scale imaging is used. Some manufacturing techniques do use multiple tests, particularly if the manufacturer is testing for different types of defects. However, it can be difficult to effectively utilize multiple tests, because the tests may not be equally reliable and often produce inconsistent results. Thus, when multiple tests are used, a “majority rules” method of combining the test results is often employed, meaning that a device is considered defective only if it fails a majority of the tests.